Nineteenth Century

32. Young’s Double Slit (1801)

Sometimes one physical theory completely overthrows and replaces another. Such a revolution occurred in the years following the period 1801–1804 when the English polymath Thomas Young (1773–1829) marshaled compelling arguments in favor of the wave theory of light. Young’s arguments were, in part, reinterpretations of data gathered a hundred years earlier by Isaac Newton and, in part, based upon his own simple experiments.

Young had to overcome a strong prejudice among natural philosophers in favor of Newton’s hypothesis that light is composed of small particles that travel in straight lines at high speeds. True, Newton had, early in his career, explored the possibility that light was composed of waves. But the waves of which he was aware (sound and water waves) tend to bend, that is, to diffract, around barriers, while light, it seems, does not. After all, we can hear but not see around corners. Consequently, Newton was drawn to the idea of particles of light. In order to explain optical phenomena more complex than the formation of shadows, Newton endowed different kinds of light particles with different tendencies to transmit and reflect. These ideas were clearly speculative, but the unqualified success of Newtonian mechanics and gravitation gave them an unearned authority.

Young broke this century-long Newtonian spell by first noting that the intensity of sound and water waves, in fact, diminishes behind the barriers around which they diffract—just as, in greater degree, the intensity of light does. Evidently, all three phenomena (light, sound, and water waves) diffract around barriers in much the same way but in different degrees. Figure 54 illustrates the geometry of one of Young’s demonstrations of diffraction—the double slit geometry. Young discussed several realizations of this geometry: one with water waves, one with light of a single color, and one with white light composed of many colors.

It is easiest to see the physics behind all three realizations when the waves are on the surface of a pool of water. Young used what today would be called a “ripple tank,” essentially a flat basin filled with water, to demonstrate how waves propagate around barriers and interact with one another. Waves are launched in the ripple tank by the periodic insertion and removal of a solid object. The dark lines in the diagram indicate the crests of these waves. The wave troughs are, of course, midway between the crests. By design, the crest of a single wave strikes the two openings in the middle barrier at the same time and launches two new waves into the area to the right. (Recall Huygens’s principle that each point on a wave front is a new point source.) Where, in the middle region, two dark lines intersect, two wave crests meet, superpose, and form a double-high crest. Where two troughs meet, a double-deep trough forms. And where a crest meets a trough, the water remains at its undisturbed level. Young coined the word interference to denote this pattern of wave interaction.

That light produces, on a much smaller scale, the same interference pattern in the double-slit geometry that water waves do is evidence that light is also composed of waves. In particular, monochromatic light originating from a single point source and propagating or diffracting through two parallel openings or slits in an otherwise opaque barrier produces a series of bright and dark bands, an interference pattern. Light, in effect, propagates along the direction indicated by the dashed lines in the diagram.

Young went on to describe the different interference patterns produced when light is diffracted around a fine thread and reflected from grooved surfaces and thin films. Young concludes one of his (typically wordy) lectures by stating that “the accuracy, with which the general law of interference of light has been shown to be applicable to so great a variety of facts, in circumstances the most dissimilar, will be allowed to establish its validity in the most satisfactory manner.” Evidently, since interference is a wave property and light can be made to produce various interference patterns, light is composed of waves.

Because Young’s printed arguments were entirely verbal, that is, without benefit of diagrams or mathematics, they were at first ignored. Eventually, Augustin Fresnel (1788–1827) provided the mathematical formulation implied by but lacking in Young’s presentations. Sometimes Young’s arguments have been interpreted as “proving Newton wrong.” If so, Young also honored Newton for his enduring contributions.

Young was a child prodigy, the eldest of ten children born to Quaker parents of modest means. He was raised by his grandfather and educated by an aunt who, for the most part, allowed him to pursue his own interests. He read with fluency by the age of two and by four had twice read through the entire English Bible. Besides European and classical languages, he studied Near Eastern ones: Hebrew, Samaritan, Chaldean, Syriac, and Persian. He kept a journal in Latin and commented on French authors in French and on Italian authors in Italian. Once when asked to exhibit his penmanship, he wrote the same sentence in fourteen different languages. While Young trained to be a physician, he also developed a serious interest in mathematics, natural philosophy (in particular, optics and botany), and various mechanical arts including telescope making.

Young’s competence in languages led him to study a copy of the three inscriptions, one in Ancient Greek, one in Egyptian hieroglyphics, and one in Demotic Egyptian, on a particular stele, the so-called Rosetta Stone, discovered in 1799 by a French officer with Napoleon’s army in Egypt. Since all three inscriptions paraphrase the same decree, the Rosetta Stone was a key to deciphering Egyptian hieroglyphics—the meaning of which had been, since the late Roman period, lost. Young’s contribution was to discern that the Demotic was a mixture of alphabetic and hieroglyphic characters and to begin the work of deciphering both Egyptian texts. When, in 1822, the French philologist Jean-François Champollion (1790–1832) independently deciphered the Demotic and hieroglyphic inscriptions, Young praised his work.

Young’s life of scholarship earned him his own stele, a memorial stone in Westminster Abbey—one that praises “a man alike eminent in almost every department of human learning, patient of unremitting labor, endowed with the faculty of intuitive perception, who, bringing an equal mastery to the most abstruse investigations of letters and science, first established the undulatory theory of light, and first penetrated the obscurity which had veiled for ages the hieroglyphics of Egypt.”

33. Oersted’s Demonstration (1820)

Once household implements began to be made of iron, people began noticing that nearby lightning strikes sometimes magnetized these implements. But what is lightning? And how does it magnetize iron? In a letter written in 1752, Benjamin Franklin described an experiment whose purpose was to answer the first of these questions. His idea was to fly a kite into a storm cloud so that any electrical charge present would be conducted down its wet string and stored in a glass container, lined inside and out with metal foil, called a Leyden jar. After explaining to his correspondent how to make a kite out of a silk handkerchief, Franklin went on to say, “And when the rain has wet the kite and twine, so that it can conduct the electric fire freely, you will find it stream out plentifully from the key on the approach of your knuckle. At this key the phial [Leyden jar] may be charged, and from electric fire thus obtained, spirits may be kindled, and all the other electric experiments be performed which are usually done by the help of a rubbed glass or tube, and thereby the sameness of the electric matter with that of lightning completely demonstrated.” One can only conclude, as Franklin does, that lightning consists of electric charges.

It was left to the Danish scientist Hans Christian Oersted (1777–1851) to address the second question: the relationship between moving charge and magnetism. The story goes that in 1820, while attempting to show his students at the University of Copenhagen that a current of moving charges had nothing to do with magnetism, Oersted placed a segment of conducting wire close to and parallel with the usual north–south orientation of a compass needle. To his surprise, when the ends of the wire were connected to a Voltaic cell or battery, the needle rotated away from the north–south line—as shown in figure 55. In actual fact, Oersted had unintentionally demonstrated that moving charges do produce a magnetic influence. The entire scene—Oersted, current-carrying wire, compass needle, and attentive students (all male)—is represented on one side of the Oersted Medal awarded annually by the American Association of Physics Teachers to an outstanding teacher of physics. Oersted’s image is certainly appropriate for this medal since his may be the only major scientific discovery made during a lecture demonstration before a class of students.

What most interested Oersted and his contemporaries was that a current of charge in the wire twists the compass needle. And, in this experimental arrangement, the twisting could not be explained in terms of an attraction or repulsion along lines joining points on the conducting wire and points on the compass needle. The only other known fundamental forces Oersted knew about, Newton’s law of universal gravitation and Coulomb’s law of electrostatics, act in this way—that is, along lines connecting two points.

Oersted found that if the current in the wire was moderately strong, the magnetic influence it produced overcame that of the earth. In this case, a set of compass needles arranged in a plane perpendicular to the wire would point in directions that circle the wire, as shown in figure 56. According to Oersted, “From the preceding facts we may likewise collect that this conflict [or influence] performs circles; for without this condition it seems impossible that the one part of the uniting wire, when placed below the magnetic pole, should drive it towards the east, and when placed above it towards the west; for it is the nature of a circle that the motions in opposite parts should have an opposite direction.” What “performs circles” around the wire was later identified as a magnetic field line. Oersted had, in effect, demonstrated that an electric current produces a magnetic field and, in so doing, initiated the study of electromagnetism.

The connection Oersted revealed between electrical and magnetic phenomena satisfied the expectations of the contemporary romantic movement—a movement that touched on all aspects of human endeavor. A romantic would see connections everywhere, and a scientifically inclined romantic would imagine that all the forces of nature were but different aspects of a single, all-encompassing, invisible power. If this power encompassed the human as well as the natural, then the natural world might suggest to us what it means to be human, and our humanity might teach us how to appreciate and care for the natural world. Oersted would have embraced these possibilities—so beautifully expressed in William Wordsworth’s sonnet The Prelude (1850).

My heart leaps up when I behold

A rainbow in the sky:

So was it when my life began,

So is it now I am a man,

So be it when I shall grow old

Or let me die!

The child is father of the man:

And I could wish my days to be

Bound each to each by natural piety.

But a romantic awareness could terrify as well as inspire. One need only read Mary Shelley’s Frankenstein (1818) or Robert Louis Stevenson’s Dr. Jekyll and Mr. Hyde (1886).

Oersted was a child of his age, a romantic, and of the last generation of scientists who called themselves natural philosophers. In addition to his work as a physicist and chemist (he was the first to isolate the element aluminum), Oersted wrote a dissertation on Kantian metaphysics and published a volume of poetry. His last work was a philosophy of life entitled The Soul in Nature (1852).

34. Carnot’s Simplest Heat Engine (1836)

Steam-driven heat engines turned wheels that, in the early nineteenth century, ground corn, wove cloth, moved goods, and lifted water out of English coal mines. By the late nineteenth century, heat engines were powering dynamos that produced electricity—that highly transportable potential to perform work. Remarkably, already in 1824 with the publication of Reflections on the Motive Power of Fire and on the Machines Fitted to Develop that Power, Sadi Carnot (1796–1832) had outlined the general possibilities and absolute limitations of heat engines.

Carnot’s Reflections is concerned not only with the theory of heat engines and their various applications, but also with the military, political, and economic implications of their development. This is not surprising given Carnot’s family background. His father, Lazare, was Napoleon’s capable general-in-chief and Sadi’s early military and scientific training was at the newly established École Polytechnique. Fatefully for Carnot, England, rather than his native France, had discovered, developed, and applied the steam engine to the point that “to take away today from England her steam-engines would be to take away at the same time her coal and iron. It would be to dry up all her sources of wealth, to ruin all on which her prosperity depends, to annihilate that colossal power. The destruction of her navy, which she considers her strongest defense, would perhaps be less fatal.”

By 1824 that “colossal power” had exiled Sadi’s father and blighted his own military career. France was worthy of a better future and a bigger role in developing the heat engine—or so Carnot must have thought. As it happened, Carnot helped create that role by developing the theory of heat engines with a precision never imagined by the English engineers of his time.

Figure 57 particularly suits the generality of Carnot’s theory. Gone are the furnaces, boilers, pistons, condensers, and smokestacks that compose a real steam engine. In his imagination, Carnot stripped away all these until he was left with only the three elements and their functions that were essential for the operation of any imaginable heat engine: a hot body that supplies heat, a device that produces work from that heat, and a cold body that absorbs waste heat. Two blocks, one circle, and three arrows represent the elements and functions that compose Carnot’s simplest heat engine.

That both a hot and a cold body are needed for a heat engine to produce work is the crucial contribution of Carnot’s Reflections. He must have been aware of the importance of this requirement because he repeated it seven times in seven successive paragraphs within the first few pages of Reflections. In Carnot’s words: “The production of motion in steam engines is always accompanied by a circumstance on which we should fix our attention. This circumstance is the re-establishing of equilibrium in the caloric; that is, its passage from a body in which the temperature is more or less elevated, to another in which it is lower.” Eliminate either the hot or the cold body, and whatever is left is no longer a heat engine capable of doing work. The logically equivalent statement “No heat engine simpler than Carnot’s simplest heat engine is possible” is a truth of the highest order—a truth that expresses what we now call the second law of thermodynamics.

Other versions of the second law of thermodynamics are better known. For example, “No process is possible whose only result is to cool a cold body and heat a hot body.” In other words, a cup of hot coffee left in a cool room never gets hotter; it always cools down. And “No process is possible whose only result is to cool a hot body and produce work.” In other words, a heat engine cannot be 100 percent efficient. The German physicist Rudolph Clausius (1822–1888) framed the first of these statements in 1850 and the English physicist William Thomson the second in 1851. Of course, Carnot’s earlier 1824 statement predates both. But each is logically equivalent to the other two. Each is a version of the second law.

One does not prove a statement that purports to be as fundamental as the second law of thermodynamics. Rather, one simply asserts that statement and from it derives important consequences. Only after these consequences have been tested and verified is the statement recognized as a fundamental law of physics.

Carnot was, indeed, able to derive important consequences from his version of the second law. For instance, he found that, in principle, “the most efficient heat engine is one that operates indefinitely slowly, without friction or dissipation, and without direct contact of hot and cold parts.” The technical word that stands for this combination of properties is reversibility. Thus, Carnot proved that “the most efficient heat engine is one that operates reversibly”—a statement traditionally called “Carnot’s theorem.”

Carnot was hampered in developing the consequences of his ideas because, at the time of writing Reflections, he did not accept the law of conservation of energy, now known as the first law of thermodynamics. (Interestingly, the second law predates the first law by more than twenty years.) In its place Carnot believed that heat, or caloric as it was called, was an indestructible fluid whose quantity was conserved as it flowed from one place to another. Not until the 1840s did James Prescott Joule’s increasingly precise experiments explode the concept of caloric and compel acceptance of the first law of thermodynamics. According to the first law, it is energy, rather than caloric, that is conserved. In this view, heat is just one way of transferring energy from one place to another. (Doing work is another.) However, such were Carnot’s gifts that, even under the fog of serious misconception, he recognized and exploited a truth of great consequence—a truth we now call the second law of thermodynamics.

35. Joule’s Apparatus (1847)

What causes things to heat up and to cool down? Thanks to the widespread use of reliable thermometers in the eighteenth century, one scientist came up with an explanation. According to Antoine Lavoisier (1743–1794), heating was caused by the flow of caloric (a “subtle fluid”) that, as it penetrated the pores of an object, raised its temperature. Caloric was thought to be ingenerate and indestructible, that is, conserved, as it flowed from one object to another. Furthermore, caloric was thought to be weightless and composed of particles that repelled one other. Hot objects were caloric-rich and cold ones caloric-poor. As caloric diffused from a hot object to a cold one, their two temperatures approached one another.

A particular amount of a particular kind of material requires a particular quantity of caloric to raise its temperatures by one degree. In this way, different objects have different heat capacities. The common substance water provides a convenient standard of comparison. By definition, one calorie is that quantity of caloric required to raise the temperature of one gram of water one degree Celsius (or centigrade). Therefore, the heat capacity of water is, by definition, one calorie per gram degree Celsius.

If caloric merely flows from one place to another, one might, with the help of a table of heat capacities, predict temperature changes in a whole class of phenomena. Pour some cold milk into hot coffee. Given the amounts of milk and coffee, their heat capacities (essentially that of water), and their initial temperatures, the final temperature of the mixture follows from the conservation of caloric. Simple calorimetric experiments done in elementary physics and chemistry labs use the same principle.

However, the concept of caloric was far from universally accepted in 1800. Benjamin Thompson’s cannon boring experiment of 1798 had seriously undermined, without altogether discrediting, the concept of a conserved caloric. Thompson (1753–1814), later known as Count Rumford, was an American original, sharp but self-aggrandizing. Born in Woburn, Massachusetts, his sympathies shifted to the British during the Revolutionary War, and when the tide turned in favor of the new nation Thompson left the rich widow he had married and resettled in England. Within a few years, King George III had knighted Thompson, and, with the king’s blessing, he became a scientific and military advisor to the elector of Bavaria all the while continuing to spy for his British patrons.

It was in this position, while supervising the boring of cannon in Munich, that Thompson, now Count Rumford of the Holy Roman Empire, reflected upon the necessity of continually cooling the cannon with water. To further investigate, he devised an experiment in which a blunt tool bore into a cylinder of brass while the whole was immersed in the water contained within a sealed wooden box. As two horses turned the bore, he notice a continual increase in the temperature of the water until, after two and a half hours, it began to boil. As Rumford noted, in a report to the Royal Society of London (1798), “It would be difficult to describe the surprise and astonishment expressed in the countenance of the bystanders, on seeing so large a quantity of cold water heated and actually made to boil without fire.” From whence came all this caloric? That caloric was released from the metal when shaved from the stock seemed unlikely since the heat capacity per mass of the metallic shavings was identical to that of the stock. Whatever its source, the supply of caloric seemed inexhaustible. According to Rumford, “It is hardly necessary to add that anything which any insulated body, ... can continue to furnish without limitation cannot possibly be a material substance: and it appears to me to be extremely difficult, if not quite impossible, to form any distinct idea of anything capable of being excited and communicated, in the manner the heat was excited and communicated in these experiments, except it be MOTION.” But Rumford’s view was not compelling. For his idea that heat is motion and stored in the motion of the smallest parts that compose a material was not easily quantified. Then again, the concept of caloric led to numerical predictions that worked—at least in calorimetric experiments. Undermining a theory is one thing. Replacing it with a sufficiently explanatory alternative is another.

It was not until 1847 that the young English brewer and amateur scientist James Prescott Joule (1818–1889) perfected a demonstration that demolished the concept of caloric, demoted calorimetry to a special case, and established the more broadly defined quantity energy as that which is always conserved. Since 1839 Joule had been laboring to show that given amounts of work generate determinate amounts of caloric—work that was variously performed by generating electrical currents, by rubbing surfaces, and by compressing gases. In each case Joule found that the same quantity of work, of whatever kind, generates the same amount of caloric. In the English units of his day, the energy required to lift approximately 780 pounds one foot generated the caloric necessary to raise the temperature of one pound of water one degree Fahrenheit. But if caloric could be generated at fixed, determinate rates, then caloric was not a conserved quantity.

Joule exhibited the apparatus depicted in figure 58 at a meeting of the British Association for the Advancement of Science in Oxford in 1847. It consisted of a well-insulated container of water into which a paddle wheel, driven by a falling weight, was inserted. Stationary vanes attached to the container kept the water from persistent circulatory motion. In this way the potential energy of the weight was dissipated in the water—the effect of which was immediately indicated by a thermometer. Joule allowed the weight to drop again and again until the temperature rose a fraction of a degree—a precision with which, as a professional brewer, he was well acquainted—and claimed that the result was reproducible. Joule’s simple design and precise measurements earned what Rumford’s experiment had not: the attention of the English scientific elite. Indeed, one of those attending the 1847 meeting was William Thomson, who in 1851 adopted conservation of energy as a postulate of the new science of thermodynamics.

Joule continued to refine his measurement of the ratio of the work to the heat it produced, now called the mechanical equivalent of heat. His last measurement of this ratio (in 1878) yielded the number 772.55, which is inscribed on his tombstone along with a verse from the gospel of John (9:4): “I must work the works of him that sent me, while it is day: the night cometh, when no man can work.”

36. Faraday’s Lines of Force (1852)

One of Albert Einstein’s earliest memories was of a compass his father had given him. Apparently, the earth, itself a large magnet, cast its influence across empty space and caused the compass needle to point north. Years later Einstein said that on holding the compass he “trembled and grew cold. ... There had to be something behind objects that lay deeply hidden.”

But is the space around a magnet really empty? Michael Faraday (1791–1867) was the first to gather evidence suggesting that what surrounds a magnet is as real as the magnet itself. He referred to this something as the “atmosphere” of a magnet or, alternatively, as its “lines of force.” Today we speak of its magnetic field.

Shortly after Hans Christian Oersted discovered, in 1820, that a wire carrying a current deflects a magnetized needle, Faraday, a self-taught English chemist and physicist, began a three-decade-long study of electromagnetic phenomena. He reported the results of his study in 3,299 consecutively numbered paragraphs that occupy some 1,100 pages of text collected in three volumes called Experimental Researches in Electricity. His final contribution to this extraordinary work is an essay, “The Physical Character of the Lines of Magnetic Force” (1852), in which he expressed his belief in the physical reality of lines of magnetic force.

It is not difficult to map a bar magnet’s lines of force with a small compass. One has only to put the magnet in the center of a large sheet of paper and position the compass nearby. Place a dot at the compass needle’s head (or north pole), shift the needle’s tail (or south pole) to the position of the dot, and place another dot at the new position of the needle’s head. Repeat this process many times and smoothly connect the dots. By convention, magnetic lines of force have a direction. They begin at a north pole and end at a south pole. The pattern of lines drawn in this way and according to this convention will approximate the idealized pattern shown in figure 59. Knowing the direction of the lines of force surrounding a magnet is equivalent to knowing in which direction a compass needle will point when placed near the magnet.

According to Faraday, magnetic lines of force belonging to different magnets have the following properties: (1) lines of force tend to shorten themselves, (2) adjacent parallel lines of force pointing in the same direction repel each other, and (3) adjacent parallel lines of force pointing in the opposite direction attract each other and then reconnect or merge. Figure 60 illustrates the arrangement and suggests the behavior of the lines of force associated with particular magnets and current-carrying wires. In the left panel, the lines of force shorten and cause north and south poles to attract each other. In the middle panel, adjacent parallel lines of force pointing in the same direction repel and cause two north poles to repel each other. Finally, the right panel shows the cross-sections of two wires, both with electrical currents flowing out of the plane of the paper. Adjacent parallel lines of force pointing in opposite directions in the region between the two wires attract each other, merge, shorten, and cause the two wires to attract.

Faraday knew that his lines of force explained nothing that could not also be explained in terms of distant objects exerting force across empty space. The lines of force are, as he admitted, “speculations” rather than deductions along a “strict line of reasoning.” Even so his lines of force are immensely satisfying. Because adjacent lines of force, or adjacent parts of the same lines, push or pull directly on each other, the lines eliminate the need for action-at-a-distance forces. Lines of force helped Faraday, and also help us, visualize what happens. While Faraday was perhaps the most productive experimental physicist of all time, his mathematical knowledge did not extend beyond elementary algebra and trigonometry. Visualizations, of which the magnetic lines of force are a prime example, did the work of mathematics for Faraday.

Faraday was born to a family so poor he often went hungry as a child. He was raised in and remained devoted to a small, dissenting, that is, non-Anglican, Christian denomination. Although he had no schooling, he was apprenticed to a kindly bookbinder who encouraged Faraday in his education. The young Faraday attended public lectures on topics in natural philosophy. These helped him, at age twenty-one in 1813, to land a menial job at the Royal Institution where eventually he was able to do his own experiments.

His invention of the electric motor (1821) and discovery of electromagnetic induction (1831–1832) brought him recognition, but his relations with the scientific establishment of his day were complicated. Although he was universally honored for his inventions, discoveries, popular lectures, and public service and he corresponded with the important scientists of his time, Faraday’s theories and speculations were generally dismissed. Faraday had no pupils and no disciples apart from James Clerk Maxwell (1831–1879). He refused a knighthood and the presidency of the Royal Society, and he declined to advise the British government on creating chemical weapons for use in the Crimean War (1853–1856). He died in 1867 before his lines of force and the concept of an electromagnetic field to which they gave birth were widely accepted.

Maxwell vindicated Faraday’s work by translating Faraday’s lines of force into mathematical language and incorporating that mathematics into a set of equations, known as Maxwell’s equations, that compose a complete theory of electromagnetism. According to this theory electric and magnetic lines of force, while produced by charges, magnets, and electrical currents, may detach from these sources and propagate with finite speed through empty space as electromagnetic waves, for instance, from Sun to Earth and from satellite to cell phone.

37. Maxwell’s Electromagnetic Waves (1865)

James Clerk Maxwell (1831–1875) so greatly admired Michael Faraday that he advised readers of his own work first to carefully study Faraday’s 1,100-page Experimental Researches in Electricity (1855). Certainly he had done so and even corresponded with its author, forty years his senior. Eventually, Maxwell paid Faraday the high compliment of constructing a mathematical model of the pictorial concept of which Faraday was most proud: his electric and magnetic lines of force.

Four equations, the celebrated Maxwell’s equations, encapsulate Maxwell’s model of Faraday’s lines of force. These equations show how electric and magnetic lines of force, or fields, are generated from their sources, that is, from charges and currents. In promoting Faraday’s lines of force, Maxwell, like Faraday before him, was at odds with the many physicists committed to the program of action-at-a-distance forces—that is, committed to explaining electromagnetic phenomena in terms of charges, in various states of rest and motion, exerting forces on other charges. Maxwell pointedly ignored forces and made fields his priority.

But a question remained. Are the lines of force and the fields to which they are equivalent real, as Maxwell and Faraday believed, or are they mere mathematical devices whose only purpose is to make convenient the calculation of forces? For a while the question remained unanswered. But in the process of constructing his equations, Maxwell discovered something Faraday had missed. Not only do magnetic fields that change their intensity, direction, or position generate electric fields as expressed by Faraday’s law (one of the four Maxwell equations), but also electric fields that change their intensity, direction, or position generate magnetic fields, an effect Maxwell incorporated into the Ampere-Maxwell law (another of the four Maxwell equations).

Together these two effects make possible self-sustaining electromagnetic waves that propagate at a speed determined by a combination of constants inherent in electromagnetic phenomena. Maxwell noticed that their predicted speed is close to measured values of the speed of light ( meters/second). Furthermore, electromagnetic waves carry energy and momentum just as light waves do. Maxwell reasonably concluded that light is composed of electromagnetic waves. In this way, Maxwell established, in 1865, a new explanation of light and endowed electromagnetic fields with physical reality. Then in 1886–1887 Heinrich Hertz (1857–1894) experimentally discovered that electromagnetic waves behave in the same way as light does and, in this way, confirmed Maxwell’s conclusion.

Figure 61 illustrates an electromagnetic wave, composed of electric and magnetic fields, each sustaining the other, and propagating in the direction shown by the thick black arrow. The whole structure looks three-dimensional, which is illusory since only one dimension of space is shown. At every point along this single spatial dimension, the electric and magnetic fields composing the wave have amplitudes, indicated by the length of their arrows, and a direction, indicated by the thick arrow’s direction. The particular waveform shown also has a definite wavelength. In general, more complex waveforms are sums of a number of waves each with its own amplitude, direction, and wavelength.

However revolutionary his discovery, Maxwell adopted the common sense of his time in supposing that electromagnetic waves require a material medium through which to propagate. After all, the waves known to Maxwell (sound waves, water waves, and waves in and on musical instruments) were waves that propagate in material media. Therefore it was natural for Maxwell and others to assume that electromagnetic waves also have a material medium—then called the lumeniferous ether. Even so, this ethereal medium avoided and continues to avoid detection. Furthermore, its properties are incoherent. The ether must be very tenuous because the planets move through it without noticeable resistance. Yet the ether must also be rigidly elastic, like steel, otherwise the speed of light would not be so high. Rather than continue to believe in a material like no other whose only purpose is to be a medium for electromagnetic waves, physicists, toward the end of the ninteenth century, simply abandoned the ether.

Maxwell, like many from nineteenth-century, middle-class English families, received his early education at home under the guidance of parents and private tutors. As a child, Maxwell was fascinated by all things that moved, made a noise, or in some way “worked” and that prompted the question “What’s the go o’ it?” or the urgent request “Show me how it doos.” He learned to draw from his cousin, Jemima Blackburn, who later became a well-known watercolorist and book illustrator.

Maxwell wrote scientific and mathematical papers before he was old enough to read them before the learned societies that published them, as was then the custom. As a young scholar he won the Adams Prize (in 1857) for an extended analysis of the stability of Saturn’s rings. Besides his contributions to electromagnetic theory, Maxwell developed a mathematical description of the range of particle velocities in a gas in equilibrium at a given temperature—the so-called Maxwell distribution—and made lasting contributions to color vision, thermodynamics, and statistical mechanics.

Maxwell died of abdominal cancer at the age of forty-eight. Maxwell’s admiring friends and colleagues mourned his early death. One of them, Professor Lewis Campbell, a friend from childhood, wrote a biography (1882) that emphasized Maxwell’s Christian faith. Maxwell, Campbell said, had taken to heart his mother’s request that he “look up through Nature to Nature’s God.”